1. Field of the Invention
The present invention relates to an apparatus for evaluating the peripheral blood flow of a living body or subject such as a patient.
2. Related Art Statement
Since the color and/or temperature of skin of a patient indicates the peripheral blood flow of the patient, an anesthesiologist checks by using his or her sensation those indications at intervals of time for identifying whether the patient, e.g., under a surgical operation has fallen in a shock due to, e.g., an abrupt decrease of blood pressure. However, since this method depends on sensations of doctors, it needs doctors'experiences and suffers from their individual differences. Additionally, since doctors use this method while referring to blood pressure reading of a blood pressure machine, it needs a considerably long time to identify a patient's shock.
In the above background of art, U.S. Pat. No. 3,769,974 discloses a reflection-type pulse wave detecting device (i.e., plethysmograph) which is basically directed to the measurement of pulse rate of a living body. The pulse wave detecting device detects the variation of intensity of the reflected light from the tissue of skin of the living body, i.e., volume pulse wave resulting from flowing of blood in the skin tissue. Since an arterial network extending in dermis under epidermis of the skin contracts when the living body falls into a shock, the volume pulse wave detected by the prior device may change (i.e., decrease) in response to the shock of the living body.
The above reflection-type pulse wave detecting device includes (a) a light emitting element which emits a light beam toward the skin, and (b) a light receiving element which is provided at a predetermined distance from the light emitting element and which receives the light which has been emitted from the light emitting element and subsequently reflected from the skin tissue. The light receiving element generates a reflected-light signal representing the reflected light received from the skin tissue. Since the reflected light from the skin tissue contains a volume pulse wave due to blood flow produced in the skin tissue in synchronism with heartbeat of the living body, the reflected-light signal from the light receiving element represents the volume pulse wave of the living body. The reflected-light signal or volume-pulse-wave signal may change due to patient's shock. However, the reflected-light signal is also influenced by other factors such as air temperature, intensity of the light beam emitted by the light emitting element, or mental condition of the patient. Thus, the reflected-light signal produced by the prior device cannot be used for identifying an abnormality of peripheral blood flow of a patient.